The following video shows American Wheat Ale yeast in active fermentation. The yeast was taken from the krausen of a beer that had been fermenting for a week. If you expand the video to fullscreen and look closely at the 400x magnification segment of the video, you will see yeast activity where small black specs are moving around inside of the yeast cell walls.
The video continues on to show the yeast at 100x and 40x magnification to give you an idea of just how many yeast cells there are on such a small glass slide. An active 5 gallon beer fermentation should have well over 10 billion active yeast cells during primary fermentation.
Video showing active yeast during fermentation:
Here is a still shot of the yeast at 400x magnification: